Loot not well concealed by sheet (7)
I believe the answer is:
pillage
'loot' is the definition.
(synonyms)
'not well concealed by sheet' is the wordplay.
'not well' becomes 'ill' ('ill' is the opposite to 'well').
'concealed by' indicates putting letters inside.
'sheet' becomes 'page' (page is a kind of sheet).
'ill' going within 'page' is 'PILLAGE'.
(Other definitions for pillage that I've seen before include "Loot" , "Sacking" , "Rob with violence" , "Plunder, sack" , "Violent stealing" .)